Nuclear fuel rods are manufactured from open-ended zirconium alloy tubular rods. One end of the rod is plugged with a zirconium alloy end plug and girth welded. Fissionable pellets are inserted into the rod, and the rod is then plugged at its open end with a conventional zirconium alloy sealing plug having a small axial opening therein for allowing gas flow in and out of the rod.
In prior art practices, the rod is transferred to a girth welding chamber where the rod is advanced against an end stop. The end stop serves two functions. It positions the fuel rod and permits evacuation of the rod for subsequent girth welding. The end stop typically includes an axial opening therethrough which communicates with the interior of the fuel rod. A vacuum is drawn through the end stop to evacuate the fuel rod to prevent oxidation when girth welding. Additionally, the end stop is fixed on bearings allowing slow rotation of the fuel rod and end stop as one unit. A tungsten inert gas welder is positioned adjacent the end stop, and as the rod rotates, the tungsten inert gas welder fuses the end plug into the fuel rod by welding the periphery of the rod end.
In accordance with prior art practices, after the fuel rod is girth welded, it is transferred to a seal weld chamber. The chamber is pressurized to approximately 600 psi with an inert gas such as argon, helium or a helium-argon mixture. As the chamber is pressurized, the fuel rod is pressurized at an equal rate through the sealing plug axial opening. When the pressure in the fuel rod is equal to chamber pressure, a welding electrode end stop is advanced adjacent and coaxial with the axial opening and an arc is discharged from the electrode to the axial opening to fuse the opening closed. Because welding is conducted in an inert gas atmosphere, oxidation and other undesirable material affects are minimized. Additionally, the fuel rod has been pressurized with an inert gas. The high pressure in the fuel rod minimizes the possibility that the rod will collapse under the high pressures attendant a nuclear reactor core. The inert gas in the rod also minimizes internal oxidation of the fuel rod.
The above described prior art practice is limited because it is expensive and cumbersome. Two separate welding chambers are used. The first chamber is evacuated for girth welding and the second chamber is pressurized. Each chamber requires separate controls and rod handling means. Different designs in end stops also are required: one for the girth welding chamber and one for the seal weld chamber, resulting in increased tooling and production costs. Additionally, the rod must be handled between the girth and seal welding operations resulting in increased handling costs. In related U.S. Pat. No. 4,837,419, both girth and seal welds are made in a single chamber, where an end stop is adapted to fill both the welding chamber and fuel rod with a gas. However, the disclosed end stop is not adapted for efficient evacuation of the fuel rod and pressurization of both the fuel rod and the welding chamber.
Therefore, it is an object of this invention to provide an apparatus wherein girth and seal welding of a sealing plug coaxially positioned in the end of a nuclear fuel rod are performed in one weld chamber.
It is another object of this invention to provide an end stop adapted for engaging a sealing plug coaxially positioned in the end of a nuclear fuel rod where the end stop permits the evacuation of the fuel rod to facilitate the girth welding of the plug into the rod, and also permits pressurization of the fuel rod and an enclosing welding chamber to facilitate the seal welding closure of the axial opening in the plug.